Optical-scanning techniques have found important applications in many areas, for example, laser printers for computers; laser direct writing lithography for production of masks, wafers and optical integrated circuits; high speed photography; IR imaging and image information transmission, graphic-art imaging for newspapers and other printed materials; and so on.
A summary of scanning methods is presented in the articles by Leo Beiser (Laser Focus/Electro-optics, Feb. 1985) and Henry E. R. Lassiter (Laser Focus World, Jan. 1991).
Optical-scanning by X-Y stage translation can only achieve low data rate because of the low translation speed of the stage. External or internal drum scanning may obtain medium data rate, but they can only be used for exposing winding materials. As a result, high resolution can not be achieved. Polygon scanning, in which the scanning trace is a straight line, can have high data rate, but the pixels per scanning line is less than 10.sup.5 or even less than 10.sup.4 since it is difficult to obtain an imaging optical system that has large angle of view as well as high resolution.
New developments of semiconducting devices are characterized by a continual decrease in the smallest dimensions and an increase in circuit size and complexity. In today's production environment, typical pixel dimensions range from 1 to 0.7 .mu.m. In laboratories, devices with structure sizes in the order of 0.5 .mu.m have been realized. In the 1990s, structures smaller than 0.5 .mu.m will be needed, e.g., for the production of 16 Mbit memory chips. The decrease in structural dimension not only permits a higher circuit complexity, but also an increase in speed and a decrease in power consumption, which is just as important.
The field of view of the objective in current projection lithographic stepper is smaller than 20 mm.times.20 mm. The linewidth d is determined by the wavelength used (.lambda.) and the quality of design and fabrication of the imaging system: ##EQU1## where K is about 0.5 to 0.9, determined by the quality of the image and NA is the numerical aperture of the objective which is normally smaller than 0.5. Because the depth of focus is inversely proportional to NA.sup.2, the optical system becomes difficult to use when NA is too large.
When a laser is used as the light source, the wavelength (.lambda.) may be shorter than 0.16 .mu.m. However, it is difficult to obtain an objective with a large field of view as well as a high resolution in extra UV wavelength due to the lack of appropriate optical material. C. W. T. Knight has recently reviewed some of these problems associated with future optical microlithography (Optics and Photonics News, Oct., 1990).
By using laser plasma, synchrotron x-ray sources or x-ray free electron laser, wavelength could be considerably shortened. Recently the development of soft x-ray multilayer coating has made soft x-ray projection lithography possible (D. L. White, et al., Solid State technology, July 1991, p37). In the past, soft x-ray was suggested for proximity lithography or projection lithography to improve system resolution and shrink structure sizes of integrated circuits. But, mask fabrication is very difficult and unstable. Moreover, soft x-ray projection systems can not achieve large area exposure. It is difficult to design a projection objective with large field of view and high resolution. Another problem is making a UV or soft x-ray optical system with a field of view larger than 20 mm and a pixel dimension smaller than 0.3 .mu.m, which is very important to the integrated circuit industry.
Recently laser raster scanning technique has been introduced for the production of masks and wafers with a resolution better than 1 .mu.m (M. Haruna et al, Applied Optics, 1987, v.26, p4587; C. Ransch et al, Applied Optics, 1989, v.28, p3754). As a direct writing lithographic method, it uses microscope objective with high resolution and small field of view, but writing speed is very slow.
Rotational scanning on planar surface on which the image spot is always focused has not become practical or useful for high speed and high resolution applications. A few rotational scanning devices are described in prior art patents: U.S. Pat. Nos. 3,588,218, 4,301,374, 3,704,372, 3,476,948, 4,413,180, 4,611,881. U.S. Pat. No. 3,746,948 discloses a room protection apparatus. U.S. Pat. No. 4,413,180 describes a robot vision system. Both devices are not imaging system and can not provide precise information of a surface during rotational scanning. U.S. Pat. No. 3,704,372 shows a rotary scan line/edge tracer using a motor driven mirror to produce the rotational scanning. There are a few problems for this device. The photocell detector is mounted adjacent the center of the stationary focusing lens which caused the loss of the light power and reduced the aperture of the lens. Furthermore, the stationary focusing lens after the rotational mirror greatly limits the field of scanning. The system can not be modified to be a recording system (e.g. for lithography) by replacing the photocell with a light source. U.S. Pat. Nos. 3,588,218, 4,301,374 and 4,611,881 describe rotational scanning devices for continuous scan in which one of the spots always scans the recording medium at any one instance. The scanner in U.S. Pat. No. 3,588,218 by Hunt et al uses an optical system with multi-focus spots which is sophisticated and technically difficult to build. To the skilled in the art, it is well known that the more complicated the optical system, the more stray light will be generated and the signal noise ratio will be decreased. Since all the spots, the image plane (the recording medium), and the planar mirror are in the same plane in the scanner, only flexible or soft recording medium can be used and because flexible recording medium such as a tape can not be made very planer, high resolution can not be achieved with such device. Moreover, because all spots are formed through different light paths which have different number of optical component, the brightness of each spot is different due to the sequential and inherent loss in light transmission and the quality of both recording and readout of data will be poor. Such difference in spot brightness will become especially significant when x-ray is used as the light source. Because the reflectivity of x-ray on the best mirror can be only about 60%, it can not be used as a light source in such devices. Schmidt (U.S. Pat. No. 4,611,881) discloses an optical apparatus for scanning radiation over a surface. A complex turnable structure with separated remote reflectors and central reflector is used. The central reflector needs to accelerate and decelerate repeatedly, which is impossible for high speed scanning required in the present art. Hashiue (U.S. Pat. No. 4,301,374) reveals an optical multi-lens scanner which is similar to some devices cited in that patent. For a four lens scanner, light source has to illuminate the entire active quarter of scanner and different part of light is allowed to pass a lens during scanning. As a result, most of the light power is wasted and this in turn greatly limits the speed of scanning because of the low exposure. The necessity of using a 50/50 mirror further impairs the light power. Moreover such illumination will cause stray light and decrease the image quality, even a shutter is used. In addition, in Hashiue's devices, the beam axis from the scanner to the detector system does not remain fixed during the scanning and the entrant direction of the light beam from the scanner to the detector changes with the rotation of the scanner. It is well known that the sensitivity of a detector is different for light coming from different direction and hence the performance of the detector in these devices will be greatly limited.